I am learning English with Duolingo.
One example sentence is:
The forks are on the plates.
Another one is:
I have her fork in my plate.
What's the difference between "in the plate" and "on the plate"?
The literal sense is...
It's not always possible to classify any given item as a plate or a bowl, but the general principle is that plates are flat surfaces, so food (or anything else) goes on them. In contrast, bowls are (also usually circular) containers with raised sides - capable of holding liquids such as soup, which goes in them.
Thus OP's example usage #1 is what we would normally expect when describing the spatial relationship between the forks and the plates.
But note the figurative sense #2 in my link - the contents of such a dish; plateful, and consider this
As a native speaker, my natural inclination is to interpret OP's second example in terms of that second definition (she sticks her fork into the food on his plate, in order to help herself to some of it).
TL;DR: Things are usually located on plates (including the figurative "I had too much on my plate already to take on another task" as per the idiomatic sense 34 in my link above). Usages such as "Lindsey stopped eating and set her fork in her plate." are relatively uncommon, so native speakers may well look for an alternative interpretation to explain/justify the unusual preposition.